1. Field of the Invention
The present invention relates to an image display apparatus and a method for manufacturing the same. More particularly, the present invention relates to an image display apparatus in which a rear plate equipped with a plurality of electron-emitting devices, and a face plate equipped with a phosphor which displays an image by being irradiated by electrons from the electron-emitting devices are disposed opposite to each other, and a method for manufacturing the same.
2. Related Background Art
There is conventionally known an image display apparatus provided with a rear plate and a face plate which are opposed to each other to be seal-bonded (see, for example, Japanese Patent Application Laid-Open No. 2000-251621). The rear plate is equipped with a plurality of electron-emitting devices in each of which an electroconductive thin film including an electron-emitting region spans a pair of device electrodes. The face plate is equipped with a phosphor for displaying an image by being irradiated by electron beams from the electron-emitting devices, and a metal-back formed on the surface of the phosphor.
On the other hand, the following methods are known as a method for manufacturing an image display apparatus (see, for example, Japanese Patent Application Laid-Open No. 2001-229828). One of the methods performs the following processes. Bake processing is performed to a face plate provided with a phosphor and a rear plate provided with electron-emitting devices for discharging impurity gases contained in these plates. A first getter processing is performed to one of, or both of the face plate and the rear plate, which have received the bake processing, and then a film of a getter material such as barium is made to adhere to the plates to be a thickness of 5 to 500 nm. After that, electron beam irradiation processing is performed to the plate processed by the first getter processing to discharge impurity gases. Moreover, a second getter processing is performed to one of, or both of the face plate and the rear plate to make a film of a getter material such as barium adhere to the plates again to be a thickness of 5 to 500 nm. After that, the face plate and the rear plate are opposed to each other to be seal-bonded. Then, the inside of an obtained image display apparatus is made to be a high vacuum state of 10−6 Pa or less. The other method performs each of the processing described above in the order of the bake processing, the fist getter processing, the seal bonding and so forth.
However, the conventional image display apparatus disclosed in the Japanese Patent Application Laid-Open No. 2000-251621 has a problem that there appear the changes of electric currents discharged from respective electron-emitting devices when the image display apparatus has performed image displaying over a long period of time of thousands of hours. Moreover, in case of the conventional manufacturing methods disclosed in the Japanese Patent Application Laid-Open No. 2001-229828, it is indispensable to form the film of the getter material to be a thickness at which the film functions as a getter. In the case where the image forming apparatus is formed by forming the film of the getter material of such a thickness onto the electron-emitting devices on the rear plate, there is produced an evil such that the electron-emitting devices do not work as the electron-emitting devices, or that the performance of the electron-emitting devices is remarkably deteriorated because reactive currents which do not contribute to electron discharging increase. Accordingly, the film of the getter material is actually formed only on the face plate which does not produce such a problem, and forming the film even on a rear plate is not performed.
Now, the changes of the electric currents in the conventional image display apparatus do not matter when all the electron-emitting devices that exist in one image forming apparatus change uniformly in the same way, for example, the electric currents uniformly increase or decrease. As a result of the present inventors' zealous research, it was found that the changes varied depending on how each electron-emitting device was driven.
To put it concretely, discharged currents increase in the electron-emitting devices which have been driven for longer times. Moreover, in the case where the drive time is the same, the discharged current increases in the electron-emitting device which performed brighter display. That is, there is a tendency of the increases in the electric currents of becoming larger in an electron-emitting device which has discharged more electrons.
Since all the electron-emitting devices cause the same increases in current when an image having the brightness over the whole screen is displayed, for example, the whole screen is displayed in white, the changes are only one which makes the whole screen brighter uniformly. However, in the image display apparatus, which is ordinarily required to display images changing from moment to moment, the homogeneity of brightness is damaged.
Moreover, in the case where a still image has been displayed for a long time in, e.g., an output screen of a computer, a part where a bright image has been displayed becomes still brighter by an increase of discharged currents, and a part where a black image has been displayed maintains the state as it is. Consequently, the distribution of brightness tends to become remarkable.
Even if the displays of different images after that are performed, the increases of the electric currents produced by the difference in the display images are not removed immediately. Then, the so-called burn-in phenomenon, in which the distribution of brightness produced by the image display till that time remains as it is, is produced, and the image display apparatus becomes one having remarkably damaged homogeneity.
As a result of a further detailed research, the present inventors found that the increases of the discharged currents are accompanied not only by the changes of the absolute values of the electric currents but also by the rises of electron discharging efficiency.
The electron discharging efficiency cited here is expressed by a ratio of an electric current which flows between device electrodes when a fixed voltage is imposed between the device electrodes of an electron-emitting device (hereinafter referred to as an “device current”) and an electric current discharged into the vacuum from an electron-emitting region (hereinafter referred to as a “discharge current”). It is needless to say that an electron-emitting device having a small device current and a large discharge current, i.e. a high electron discharging efficiency, is preferable as an image display apparatus.
In the case where a discharge current increases owing to the above-mentioned image display, since the increase in the device current is not large in comparison with the increase in the discharge current, the electron discharging efficiency of the electron-emitting device itself rises as a result. Like the above-mentioned increase of the electric current, by a drive for a longer time or by a brighter image display, this change also becomes remarkable, and further a burn-in phenomenon is also produced.
Both of these increases in the electric current and the electron discharging efficiency became the factor which deteriorates the homogeneity of a display image, and they have been serious obstacles for realizing an image display apparatus which is required to perform a high definition image display over a long period of time.